Recently, stainless steel for wear-resistance applications is receiving attention in industrial fields as an alternative to high-strength carbon steel. The reason why attention is paid to wear-resistant stainless steel is that high-strength carbon steel has to be frequently replaced because of the poor corrosion resistance thereof. Particularly in the oil sands industry, the demand for wear-resistant material suitable for the purification and transport of oil sands is increasing. Such wear-resistant stainless steel for industrial equipment should typically have high hardness, and should be resistant to intergranular corrosion at welds. Furthermore, minimum impact properties are required to ensure equipment stability.
Generally, stainless steels are classified depending on the chemical components or the metal structure thereof. Depending on the metal structure, stainless steels are classified into austenitic (300 series), ferritic (400 series), martensitic, and duplex stainless steels.
Among these stainless steels, ferritic (400 series) stainless steels have superior processability and corrosion resistance. In particular, 410 series steel is composed mainly of 0.15 wt % or less of C and 11 to 13 wt % of Cr. The use of high C content is advantageous because high hardness may be obtained through thermal treatment. However, 410 series steel is disadvantageous because the base material and welds have poor low-temperature impact properties, and also because intergranular corrosion is very severe at welds due to the absence of stabilizing elements such as Ti or Nb to ensure high hardness.
Hence, there is the need for wear-resistant material having superior impact properties and sufficiently high hardness, despite containing stabilizing elements, in order to apply it to wear-resistant equipment.
Currently widely available as stainless steel containing low Cr (11 to 13%) with superior impact properties at welds is 3Cr12 steel, containing 11.5% of Cr with Ti. 3Cr12 steel is configured such that about 0.025% C-11.5% Cr is added with small amounts of Ni and Mn, and thus the heat affected zone of welds has a dual phase of ferrite and martensite, to thereby improve impact properties of welds.
In particular, U.S. Pat. No. 4,608,099 (Patent Document 1) discloses steel in which Ti is removed from 3Cr12 steel and Mo is added in an amount of 0.2 to 0.5% to further improve the impact properties of the base material of 3Cr12 steel. The steel disclosed in Patent Document 1 is used through thermal treatment at an annealing temperature of 670 to 730° C., and thus exhibits a yield strength of 303 to 450 MPa and a tensile strength of 455 to 600 MPa, ultimately resulting in high strength compared to typical ferritic stainless steel. However, this steel has low softness and is thus unsuitable for use in wear-resistance applications. Such steels manifest low Brinell hardness of about 140 to 180 HB, and are thus inappropriate for wear-resistance applications. Moreover, there are problems with low corrosion resistance because of the precipitation of Cr-carbide in the heat affected zone of welds, attributable to the absence of stabilizing elements such as C and N.